Electret articles comprise a dielectric material exhibiting a persistent or quasi-permanent electrical charge. See G. M. Sessler, Electrets, Springer Verlag, N.Y. (1987). The articles are commonly used in the form of fibrous filtering webs for applications, and processes for making electret nonwoven fibrous filter webs are very well known. For example, nonwoven webs can be made from polymers using melt-blowing techniques, such as those described in Van Wente, "Superfine Thermoplastic Fibers," Ind. Eng. Chem., vol. 48, pp. 1342-46, (1956), and an electric charge can be imparted in the web using various techniques. (See U.S. Pat. Nos. 4,215,682; 4,588,537; 5,411,576 and 5,472,481; 5,645,627; 5,496,507; and WO 97/07272).
Because of the importance of air filtration and the desirable properties that electret filter webs have shown in filter applications, considerable efforts have been devoted to improving the performance of fibrous electret filters. The above-cited patents reflect some of the work that has been reported to improve electret filter performance, and what follows is a brief summary of these contributions.
Kubik and Davis in U.S. Pat. No. 4,215,682 imparted an electric charge in melt-blown fibers by bombarding the fibers with electrically charged particles as the fibers issued from a die orifice.
Klaase et al. in U.S. Pat. No. 4,588,537 injected charge into an electret filter using a corona treatment.
Jones et al. in U.S. Pat. Nos. 5,411,576 and 5,472,481, disclose electret filters that are made by extruding a blend of polymer with a melt-processable fluorochemical in a microfibrous web. The resulting web is annealed and corona treated.
Lifshutz et al. in U.S. Pat. No. 5,645,627 (WO 96/26783) makes electret filters by extruding a blend of polymer with a fatty acid amide or a fluorochemical oxazolidinone or a mixture of these, in a microfibrous web, followed by annealing and corona treating the resulting web.
Angadjivand et al. in U.S. Pat. No. 5,496,507 indicate that impinging water droplets onto a nonwoven microfiber web imparts a charge to the web.
Rousseau et al. in WO 97/07272 disclose electret filters that are made by extruding blends of a polymer with a fluorochemical or organic triazine compound into a microfiber web, followed by impinging water droplets onto the web. This publication indicates that use of these additives results in improved charge when the web has been impinged by water droplets.
Although the above documents disclose a variety of methods for improving electret filter performance, the previous efforts have nonetheless left room for further contributions and the invention described below is yet another discovery directed toward the ongoing effort of establishing better electret fibers and filters.